Simultaneous optical and multi-band terahertz imaging using an atomic quantum sensor

TL;DR

The paper addresses the challenge of rapid, multispectral terahertz imaging by introducing a two-species atomic-vapour sensor that converts two discrete THz frequencies into optical fluorescence, enabling simultaneous imaging at $0.549$ THz and $1.012$ THz with an optical overlay. Using a shared rubidium–cesium vapor cell and optically transparent THz lenses, the system maps THz fields to narrow optical transitions, producing fluorescence at $495$ nm (Rb) and $535$ nm (Cs) that are captured by separate cameras, while an optical channel records a standard image. Image processing combines THz-on/off ratios and brightfield normalization to form a widefield $42\,\mathrm{mm} \times 42\,\mathrm{mm}$ view, and a Cs–Rb fluorescence vector map in $|R|$ and $\phi$ is color-coded in an HSL space to reveal spectral differences, demonstrated here on sugar samples (glucose, maltose, lactose, PTFE). The work demonstrates simultaneous multispectral THz imaging with optical overlay, highlighting potential for integrating THz sensing into optical instruments and enabling rapid, non-destructive materials analysis with a hybrid quantum sensor. Practical implications include faster, label-free material discrimination and the prospect of new hybrid sensing devices leveraging atom-based THz detection.

Abstract

We demonstrate simultaneous imaging at 0.549 THz and 1.012 THz with an optical overlay using a two-species atomic-vapour-based technique. The atomic vapour, comprising laser-pumped rubidium and caesium atoms contained within the same cell, is used to convert two narrowband terahertz signals to optical frequencies which can then be detected using standard CMOS sensors. We use the system to image and perform spectral analysis of material samples. As atomic vapour is optically transparent, by using optically-transparent terahertz lenses, we can achieve simultaneous optical imaging, allowing for potential integration of terahertz sensitivity into a range of optical imaging devices.

Figures (3)

• Figure 1: Experimental setup of the two-species imager (a). Two fields at 0.549 (red) and 1.012 (blue) are collimated with f=75 mm lenses, and then overlapped by a high resistivity float zone (HRFZ) Si beamsplitter such that the beams are co-linear. This combined bi-chromatic field interacts with a target object, and is then re-imaged by a polymethylpentene (PMP) lens relay such that light from the target plane is refocussed into the science cell at the position of a light sheet (orange). This light sheet excites the ${}^{85}$Rb & ${}^{133}$Cs atoms to states with transitions that are resonant with the 1.012 & 0.549 fields. In the presence of these fields, the atoms fluorescence at 495 (c) and 535 (d), respectively. Real-colour images of the optical fluorescence when imaging a '$\Psi$' pattern from a metal test card (b) are shown in ${}^{85}$Rb (e) and ${}^{133}$Cs (f).
• Figure 2: A schematic of the spectral separation of the fluorescence emission from atomic vapour in the science cell (a). The excited ${}^{133}$Cs and ${}^{85}$Rb atoms fluoresce at 535 nm and 495 nm light in response to the presence of 0.549 and 1.012 fields, respectively. Dichroic mirrors (dash cyan & green lines) are used to direct wavelength-bands (coloured regions) containing the Rb and Cs fluorescence peaks towards cameras 1 & 2, while the remaining optical light is captured by camera 3 (b). These wavelength bands are filtered further with narrow-band filters, with the resulting data captured by each camera and processed is shown in (c--e).
• Figure 3: Individual THz fields at 0.549 (a) and 1.012 (b), are used as X and Y co-ordinates, respectively, in a vector space (c). For each pixel pair, the resulting vector phase and magnitude is mapped to an HSL colour map. The resulting phasemap coloured by pixel phase (d), with the corresponding optical image is shown in (e).